Optimization of efficiency and energy density of passive micro fuel cells and galvanic hydrogen generators
📝 Abstract
A PEM micro fuel cell system is described which is based on self-breathing PEM micro fuel cells in the power range between 1 mW and 1W. Hydrogen is supplied with on-demand hydrogen production with help of a galvanic cell, that produces hydrogen when Zn reacts with water. The system can be used as a battery replacement for low power applications and has the potential to improve the run time of autonomous systems. The efficiency has been investigated as function of fuel cell construction and tested for several load profiles.
💡 Analysis
A PEM micro fuel cell system is described which is based on self-breathing PEM micro fuel cells in the power range between 1 mW and 1W. Hydrogen is supplied with on-demand hydrogen production with help of a galvanic cell, that produces hydrogen when Zn reacts with water. The system can be used as a battery replacement for low power applications and has the potential to improve the run time of autonomous systems. The efficiency has been investigated as function of fuel cell construction and tested for several load profiles.
📄 Content
9-11 April 2008 ©EDA Publishing/DTIP 2008
ISBN: 978-2-35500-006-5 Optimization of efficiency and energy density of passive micro fuel cells and galvanic hydrogen generators
Robert Hahn, Stefan Wagner, Steffen Krumbholz, Herbert Reichl,
Fraunhofer IZM, Gustav- Meyer-Allee 25,
D-13355 Berlin, Germany
Abstract- A PEM micro fuel cell system is described which is based on self-breathing PEM micro fuel cells in the power range between 1 mW and 1W. Hydrogen is supplied with on-demand hydrogen production with help of a galvanic cell, that produces hydrogen when Zn reacts with water. The system can be used as a battery replacement for low power applications and has the potential to improve the run time of autonomous systems. The efficiency has been investigated as function of fuel cell construction and tested for several load profiles.
I.
INTRODUCTION
During the last few years, the development effort
related to small, portable fuel cells has increased
significantly. The main motivation underlying the
development of micro fuel cells is the possibility to
achieve higher energy densities compared to batteries.
This development benefits greatly from the existing
knowledge and attempts to improve larger fuel cells for
automotive, residential, and stationary applications. For
the commercialisation of both big and small fuel cell
systems, however, improvements still are required in
several areas. For DMFCs, for example, it has been
recognised that the success of this fuel cell technology
depends largely on developing better membranes with
lower methanol cross-over and improving the electro-
catalysts which can overcome the slow anode kinetics.
When developing smaller fuel cells, it is
impossible
to
simply
use
scaled-down
systems
architectures and components applied in their larger
counterparts.
A complete portable fuel cell system consists of three
major parts:
The fuel cell stack which is the core of the system. Its
size is related to the power output.
The fuel tank. Its size is related to the amount of
stored energy and, hence, to the runtime of the device.
The balance of plant (BOP) which includes all the
peripheral components that support the power generation
process. In most cases, this is the hydrogen-generating
system for PEM fuel cells.
Since compressed gas or liquid hydrogen cannot
be used for portable or small fuel cells, the research
focuses on three kinds of fuel cell. The first are the direct
liquid fuel cells using methanol (DMFC), ethanol
(DEFC) or formic acid (DFAFC). Then, the PEM fuel
cells with hydrogen are considered, where the hydrogen
is generated from reformed methanol, reversible storage
alloys or chemical hydrides and water-reactive alloys.
The last type of interest, but still in the state of basic
research is the biofuel cell. In this case, organic materials
like alcohols, organic acids or glucose are used as a fuel
and biocatalysts convert chemical into electrical energy.
So far, the existing prototypes have shown a very low
power density and short lifetime. Therefore, they will not
be examined here.
Many attempts have been made to date to reduce the balance of plant of portable fuel cells in order to increase reliability and reduce costs. The related studies revealed that for portable fuel cells sophisticated peripheral components have to be developed to allow for a higher power density and operation under varying loads and ambient conditions. The key challenge in this field is how to achieve the desired power performance, while simplifying the design of the BOP in order to miniaturise the whole system. With miniaturisation, application-specific components like valves and pumps based on micro systems technology have to be developed.
A. The fuel cell core – micro fabrication technologies
Typically, large fuel cells are mechanically compressed sandwiches of a graphite composite or metal electrodes and membrane assemblies. Each component of the fuel cell has to be re-designed based on well- established technology platforms for miniature components in order to achieve a cost-effective miniaturisation. Therefore, most researchers use available manufacturing techniques like:
Silicon and MEMS technologies 9-11 April 2008 ©EDA Publishing/DTIP 2008
ISBN: 978-2-35500-006-5
Foil processing of polymer and metal foils, polymer substrates
Printed-circuit board technology
Planar ceramic technology like low-temperature co-fired ceramics (LTCC).
The advantage of printed-circuit board technology as a basis of flow field and current collector fabrication above all is the low-cost mature technology. Furthermore, light-weight and stiff composite materials are used and design flexibility is ensured, as complex conductor/insulator patterns are applied either as a mono- or multi-layer design. However, the standard material like the copper/glass epox
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